The present invention is concerned with processes for making dehydration products from sugar alcohols, and more particularly but without limitation, to acid-catalyzed processes for making isohexides, such as isosorbide, from hexitols such as sorbitol or from monoanhydrohexitols such as 1,4-sorbitan.
The dehydration products that can be made by the acid-catalyzed double dehydration of sugar alcohols (that is, by removing two waters in succession from a sugar alcohol), in particular, hexitols such as sorbitol, have been the subject of extensive work. Isosorbide, also known as 1,4,3,6-dianhydrosorbitol, is now commercially produced and marketed as a monomer for imparting renewable content to polyesters and polycarbonates, and has been used as a pharmaceutical intermediate.
A variety of acid catalysts have been evaluated for use in carrying out the dehydration of sorbitol through certain monoanhydrohexitol intermediates (e.g., 1,4-sorbitan) isosorbide. Inorganic acids such as H2SO4, H3PO4, and HCI are readily obtained, inexpensive materials but are difficult to regenerate. In order to avoid the regeneration and attendant disposal problems, solid resin catalysts have been tried. Unfortunately, in the presence of water and at the temperatures required for carrying out the dehydration, very few solid acids can demonstrate the activity and stability needed to begin to contemplate a commercially viable process.
U.S. Pat. Nos. 6,849,748; 7,420,067; 7,439,352; 7,772,412 and 7,982,059 provide examples of prior art methods for producing isohexides (also referred to as anhydrosugar alcohols, anhydrohexitols, anhydroalditols etc) such as isosorbide, from sorbitol from dextrose.
Commonly-assigned U.S. Pat. No. 6,849,748 to Moore at al., for example, describes a solvent-free process wherein a sugar alcohol—such as sorbitol—is heated with stirring until molten, and then dehydrated in the presence of a soluble acid or acidic on exchange resin with stirring, under vacuum (to remove the water product and drive the reaction toward the products) and at an elevated temperature, then the resulting anhydrosugar alcohol is purified by distillation, followed by melt crystallization and/or redistillation. The final, purified product is isolated by centrifugation or filtration. Enumerated preferred acid catalysts include sulfuric acid, phosphoric acid, p-toluenesulfonic acid and p-methanesulfonic acid.
Commonly-assigned U.S. Pat. No. 7,420,067 mentions these same acids, as well as acidic ion exchange resins and acidic zeolite powders as additional options. Successive film evaporators, especially wiped film evaporators under vacuum, are described for use in purifying the product isosorbide.
U.S. Pat. No. 7,772,412 to Holladay et al. describes a process for making isosorbide wherein sorbitol is fed to as reactor containing a dehydration catalyst and a hydrogenation co-catalyst, with hydrogen being supplied countercurrently to the reactor for removing water as it is formed and for “reducing or eliminating . . . oligomeric or polymeric material in the dehydrator product”, to which undesirable color formation had been attributed. Suitable dehydration catalysts include the mineral acid catalysts, solid acid catalysts such as the heteropolyacids, mesoporous silicas, acid clays, sulfated zirconia, molecular sieve materials, cation exchange resins and zeolites, and combinations of any of these. The hydrogenation catalyst is described as typically being a supported metal or multi-metal catalyst. Palladium in particular is described as especially preferable for the metal, with platinum, nickel, cobalt, ruthenium, rhenium, rhodium, iridium and iron also being listed.
U.S. Pat. No. 7,982,059 describes a process for converting aqueous sorbitol to xylitol and isosorbide in the presence of an acid catalyst and without a hydrogenation co-catalyst, more particularly involving reacting an aqueous sorbitol solution with an acid zeolite at about 250 degrees Celsius and a pressure maintained at from about 68 bars to about 80 bars to produce the xylitol and isosorbide.
WO 2013/138153 to Binder et al. describes a process for forming one or more dehydration products from an aqueous sugar alcohols solution including one or more alcohols from pentoses and hexoses, wherein the aqueous sugar alcohols solution is subjected to an acid-catalyzed dehydration using a substituted sulfonic acid catalyst solubilized in the aqueous sugar alcohols solution. In certain preferred embodiments, the dehydration process is conducted rapidly and with rapid cooling of the dehydration products prior to any separation of the residual sugar alcohol(s) from the dehydration products in the overall product mixture, in the manner prescribed for the dehydration of aqueous sugar solutions in WO 2013/106136 to Sanborn et al.